Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming (which is sometimes referred to as writing) of charge storage structures (e.g., floating gates or charge traps) or other physical phenomena (e.g., phase change or polarization), determine the data value of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, cellular telephones, solid state drives and removable memory modules, and the uses are growing.
A NAND flash memory device is a common type of flash memory device, so called for the logical form in which the basic memory cell configuration is arranged. Typically, the array of memory cells for NAND flash memory devices is arranged such that the control gate of each memory cell of a row of the array is connected together to form an access line, such as a word line. Columns of the array include strings (often termed NAND strings) of memory cells connected together in series, source to drain, between a pair of select lines, a source select line and a drain select line. A “column” refers to a group of memory cells that are commonly coupled to a local data line, such as a local bit line. It does not require any particular orientation or linear relationship, but instead refers to the logical relationship between memory cell and data line. The source select line includes a source select gate at each intersection between a NAND string and the source select line, and the drain select line includes a drain select gate at each intersection between a NAND string and the drain select line. Each source select gate is connected to a source line, while each drain select gate is connected to a data line, such as column bit line.
As memory densities increase, issues with data retention tend to worsen. For example, while polysilicon (sometimes referred to as polycrystalline silicon) charge storage structures have been commonly used for years, they tend to become impracticable as their thickness becomes too thin, e.g., around 6 nm or less. The practicality of polysilicon charge storage structures can be improved through the incorporation of low-concentration metal impurities within the polysilicon, e.g., on the order of 1E20 atoms/cm3 or less. However, conventional methods of doping such metal impurities are thought to present their own challenges. For example, in beam-line implantation of impurities, low energy levels would be necessary for such low-level implantations, which may not be viable for cost-effective implantation rates. Furthermore, in plasma doping, it is difficult to form a plasma source from a pure metal, and additional impurities, such as carbon, of the metal source gases could lead to undesirable levels of these impurities in the polysilicon. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative methods of incorporating impurities.